1. Field of the Invention
The present invention relates to an optical information recording/reproducing system which executes focusing and tracking controls while selectively adjusting the loop gain of a servo feedback system.
2. Description of the Prior Art
Optical information recording/reproducing systems record information data in binary code on an information recording medium depending upon a physical change in such a way that a focused light beam is irradiated upon the medium. Minute recesses, so-called pits, are formed on the recording layer surface of the medium. Also, the magnetization direction is changed on a recording layer surface of a medium which is made of ferromagnetic substance. The generated physical change, representative of recorded information, is reproduced by optically detecting the light reflected from or transmitted through the medium.
In general, a sufficiently strong light beam is required to generate the above-mentioned physical change required in data recording or erasing operation. On the other hand, a light beam not strong enough to generate the physical change is required to reproduce data in the data reproducing operation. In recording information data, predetermined information is first read out from preformat parts where address information has previously been written (when the medium was being formed) and then data is recorded at groove parts, where data have not yet been written on or along tracks arranged on the recording medium. Therefore, it is essential to execute precise tracking control so that a light beam always moves exactly along the groove parts on the guide track in both recording and reproducing operations. Further, it is necessary to execute precise focusing control so that the light beam can always be projected onto the medium sharply in focus.
In general, the above-mentioned tracking control and focusing control are executed under closed loop control. That is, the light obtained when the projected light beam is reflected from or transmitted through the recording medium is detected by a twice or four times-divided photodiode, and an optical system is controlled by feeding back the detected optical signal. In the case of tracking control, in practice, the optical system is controlled in such a way that control current is passed through an actuator for an object lens (or an object mirror) to move the lens in the direction parallel to the medium, that is, to move a carriage on which the lens is mounted by controlling a voice-coil motor. On the other hand, in the case of focusing control, a voice-coil motor is driven to move an optical pickup in the direction perpendicular to the medium or a lens is moved in the direction perpendicular to the medium. These controls are achieved on the basis of the detection signal obtained from the reflected or transmitted light. However, since the intensity of light reflected from or transmitted through the recording medium varies according to various conditions, when the detection signal is simply fed back at a constant feedback ratio in the closed loop control system, the control inevitably becomes unstable due to loop gain variations. The most distinguishable example of difference in the intensity of the reflected or transmitted light is a difference in light beam intensity between recording operation and reproducing operation. This is because the light beam intensity in recording operation is far greater than that in reproducing operation. Therefore, in the case where the detection signal proportional to the reflected or transmitted light intensity in voltage level is simply fed back as it is, the feedback ratio in recording mode is far greater than that in reproducing mode, thus resulting in a problem in that it is impossible to achieve a stable closed loop control. To improve such shortcomings, Japanese Published Unexamined Patent Application No. 56-145533 discloses a control method or system provided with means for adjusting the gain of a closed loop in such a way as to decrease the gain in recording mode and to increase it in reproducing mode, that is, to reduce the difference in feedback ratio between recording and reproducing operations.
However, in the prior art system described above in which only the loop gain is adjusted in recording and reproducing operations, there still exists a problem in that it is impossible to perform a fine gain adjustment, although the loop gain can be adjusted coarsely. This is because the intensity of the reflected light is not constant but changes even during the reproducing operation. The intensity of reflected light is different between when the light beam is being projected upon the preformat part and when it is projected upon the groove part or the data part. Therefore, it is impossible to accurately control the position of the light beam, thus readily causing an offset of light beam travel deviating from the middle position of the recording pits. The above problem eventually lowers the S/N ratio and produces errors in recording or reproducing information data in or from the recording medium, thus reducing the reliability of the system.
Further, in this specification, the above groove part implies an area at which information data has not yet been written and the above data part implies an area (the same position as the groove part) at which information data has already been written.